Magnetic resonance imaging (MRI) is a common and well known technique for imaging the internal structure of objects and for medical diagnosis. Conventional MRI requires that the object to be imaged be placed in a homogeneous (typically to within 40 ppm) and strong (typically in the range of 0.5 to 1.5 Tesla) magnetic field. Generating such magnetic fields is difficult and expensive.
Prepolarized MRI (PMRI) is a recent technique which uses a strong, nonhomogeneous pulsed magnetic field in combination with a weaker, homogeneous magnetic field to perform imaging. The strong, pulsed field is known as the polarizing field and it is produced by a polarizing magnet. The weaker, homogeneous field is known as the readout field and is produced by a readout magnet. PMRI is also referred to as switched-field MRI and is related to field cycling nuclear magnetic resonance (NMR) relaxometry.
In PMRI, the polarizing field is switched on briefly (about 0.01 to 2 seconds) to polarize the nuclear spins inside the object to be imaged. Then, the polarizing field is rapidly reduced at a rate faster than the decay rate of the nuclear spin polarization. The nuclear spin polarization is then analyzed in the readout magnetic field. The polarizing field causes the nuclear spin polarization to be greater than it would be with only the readout field. Reference can be made to U.S. Pat. Nos. 5,629,624 to Carlson et al., 4,906,931 to Sepponen, and 5,057,776 to Macovski concerning PMRI.
A result of the pulsed polarizing magnetic field is that it renders a large PMRI device very difficult to build. The magnetic energy stored in the magnet must be removed and restored with every pulse. This practically limits the amount of energy which can be stored in the pulsed magnet and thus the size of the PMRI device. Therefore, future PMRI devices will likely be small dedicated imagers, dedicated to imaging small body parts such as hands, feet, knees, heads, breasts, neck and the like.
Imaging small body parts places limitations on magnet geometry (both readout and polarizing). Most body parts are not cylindrical and therefore do not efficiently occupy the volume inside a traditional cylindrical magnet assembly. A cylindrical magnet assembly is a collection of coils arranged on the surface of a cylinder. Access to the magnetic field of a cylindrical magnet is limited to the end openings of the cylinder or between the coils. This limited access makes it difficult and uncomfortable to image certain body parts such as knees. Of particular difficulty is providing a magnet for imaging a knee or elbow as it is flexing.
It would be an advance in the art to provide readout magnet designs which allow increased access to the homogeneous magnetic field. Such improved magnet designs would be particularly well suited for use in dedicated PMRI machines.
Lee-Whiting discloses a design for a 4-coil biplanar magnet in "Homogeneous Magnetic Fields", Atomic Energy Commission of Canada Limited CRT-673, 1-29 (1957). The magnet has 2 coils symmetrically and coaxially disposed in each of two parallel planes. The homogeneous magnetic field is located between the planes defined by the coils. The homogeneous magnetic field can be accessed from the radial direction (i.e., from between the planes defined by the coils). A similar 4-coil design is also disclosed by Garrett in "Thick Cylindrical Coil Systems with Field or Gradient Homogeneities of the 6th to 20th Order", Journal of Applied Physics, 38, 2563-2586 (1967). A shortcoming of these 4-coil designs is that they are relatively inefficient in producing the desired homogeneous field, and produce relatively inhomogeneous magnetic fields.
U.S. Pat. No. 4,829,252 to Kaufman discloses an MRI system with improved patient access to the magnetic field. The system of Kaufmann uses biplanar magnets to produce the required homogeneous magnetic field. Kaufman does not disclose how to design the biplanar magnets or specific biplanar magnet designs.